The present invention generally relates to ion implanters and more particularly, relates to an apparatus and a method for calibrating the zero-angle position of a wafer platform in an ion implanter.
Ion beam implanters are used to implant or xe2x80x9cdopexe2x80x9d silicon wafers with impurities to produce n or p type doped regions on the wafers. The n and p type material regions are utilized in the production of semiconductor integrated circuits. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type material. If p type material is desired, ions generated with source materials such as boron, gallium or indium are typically used.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The beam is formed and shaped by apparatus located along the beam path en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
During ion implantation a surface is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. The size of the wafer or substrate (e.g. 8 inches or greater) is typically much larger than the cross-section of the irradiating beam which deposits on the wafer as a spot or xe2x80x9cribbonxe2x80x9d of about 1 inch. Commonly, in high current machines, the required uniform irradiance is achieved by moving the wafer through the beam.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter beam forming and shaping structure adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials also include undesirable species of ions generated in the ion source, that is, ions having the either the wrong atomic mass or undesired ions of the same atomic mass.
In a conventional ion implanter 10 such as that shown in FIG. 1, an ion beam 12 is emitted from an ion source 14 and passed through a pre-analyzing magnet 16 to remove undesired types of ions. Ions having identical energies but different masses experience a different magnetic force as they pass through the magnetic field due to their differing masses thereby altering their pathways. As a result, only those desired ions of a particular atomic mass unit (AMU) are allowed to pass through a pre-positioned orifice in the pre-analyzing magnet.
After passing through the pre-analyzing magnet the ion beam is accelerated to a desired energy by an accelerator 18. Negative ions are changed into positive ions by a charge exchange process involving collisions with a chemically inert gas such as argon. The positive ions then pass through a post-analyzing magnet (not shown), and a pair of vertical and horizontal scanners 20,22 finally reach a wafer 24 where they impact the wafer 24 and are implanted.
Ion implantation has the ability to precisely control the number of implanted dopant atoms into substrates to within 3%. For dopant control in the 1014-1018 atoms/cm3 range, ion implantation is superior to chemical diffusion techniques. Heavy doping with an ion implanter, for example, can be used to alter the etch characteristics of materials for patterning. The implantation may be performed through materials that may already be in place while other materials may be used as masks to create specific doping profiles. Furthermore, more than one type of dopant may be implanted at the same time and at the same position on the wafer. Other advantages include the fact that ion implantation may be performed at low temperature which does not harm photoresist and in high vacuum which provides a clean environment.
A sample mounting stage 30 for the ion implanter 10 is shown in FIG. 2. The sample mounting stage 30 is constructed with a wafer platform 32 for positioning a wafer thereon. The wafer platform 32 may be equipped with electrostatic chucking (ESC) device or be provided with a mechanical clamping device. The wafer platform 32 is controlled by the mechanical motion of the rotating head 34. During the normal operation of an ion implanter, the sample mounting stage 30 must be frequently calibrated. Particularly, the wafer platform 32 for mounting of a wafer thereon must be accurately calibrated such that when a wafer (not shown) is positioned on the wafer platform 32, it is parallel with the ion beam from the ion planter. Prior to the implantation process, the wafer must first be clamped, or otherwise affixed to the wafer platform 32 and then rotated by the rotating head 34 to 90xc2x0. After the rotation or tilt of 90xc2x0, the ion beam from the ion implanter and the normal line of the wafer are parallel, and thus the commonly known term of xe2x80x9czero-anglexe2x80x9d. After repeated usage of the ion implanter, and the repeated rotation of the wafer platform 32, the angle of the wafer platform may be displaced with an error and therefore, the position of the wafer platform must be calibrated and corrected.
A commonly used implantation angle between the ion beam and the wafer surface is about 7xc2x0 such that a channel effect can be avoided to eliminate errors caused by inaccurate depth of implantation. Under other operating conditions, for instance, at large angle implantation for the sidewalls or under a gate, the ion implanter may need constant calibration in order to carry out an accurate implantation process. A most frequently used base point for calibration is the zero-angle formed between the ion beam and the normal line of the wafer. The calibration can be advantageously carried out as long as the zero-angle between the ion beam and the normal line of the wafer is achieved.
During an attempt to calibrate the wafer platform 32 to a zero-angle position, it was discovered that the machine supplier for a medium density ion implanter does not supply a calibration tool which can be used to accurately calibrate the positioning of the wafer platform. For instance, a crude method for determining the calibration of the wafer platform is to measure the distance between the wafer edge and the support wall of the implanter. When an unequal distance is measured, the screws that fixed the position of the wafer platform are removed and then, the distances are adjusted until a ruler measures equal distance to the support wall from the wafer edge. The method is crude and difficult to execute. Its reproducibility is poor between different machine operators.
It is therefore an object of the present invention to provide a calibration method for a medium current ion implanter that does not have the drawbacks or the shortcomings of the conventional method.
It is another object of the present invention to provide an apparatus for calibrating the zero-angle of a wafer platform in a medium density ion implanter which can be used to produce reproducible result.
It is a further object of the present invention to provide an apparatus for calibrating the position of a wafer platform in a medium current ion implanter that is constructed of a curvilinear piece supported by two linear rods.
In accordance with the present invention, an apparatus and a method for calibrating the position of a wafer platform in a medium current ion implanter are provided.
In a preferred embodiment, an apparatus for calibrating the position of a wafer platform in an ion implanter is provided which includes a curvilinear piece formed of a first rigid material, the curvilinear piece has a half-circular shape, a predetermined thickness, a predetermined depth, an inside diameter, an inside peripheral surface, a first open end and a second open end, the inside diameter is substantially equal to an outside diameter of the wafer platform such that the inside peripheral surface intimately engages an outside peripheral surface of the wafer platform when the wafer platform is properly calibrated at a zero-angle position; a first and a second linear rod formed of a second rigid material that has equally a first length, a rectangular cross-section, two front ends and two backends, the two front ends have a thickness substantially equal to the predetermined thickness of the curvilinear piece for mounting the curvilinear piece thereon by engaging the first and second open ends of the curvilinear piece, the curvilinear piece may have a plane along radial direction that is perpendicular to a plane of the first and second linear rod along a longitudinal direction when the curvilinear piece is mounted to the first and second linear rod, the first length is substantially equal to a distance between a plane of the wafer platform and a plane of an ion emitter for the implanter such that when the position of the wafer platform is properly calibrated at a zero-angle position, the first and second linear rod fit snugly on the implanter; and at least one cross-bracing rod connected in-between and juxtaposed to the two backends of the first and second linear rod for providing rigidity to the apparatus.
In the apparatus for calibrating the position of a wafer platform in an ion implanter, the first and second rigid materials are selected from a group consisting of aluminum, stainless steel and fiber reinforced plastic. The inside diameter of the curvilinear piece is equal to the outside diameter of the wafer platform. The at least one cross-bracing rod may be two cross-bracing rods with one connecting the two backends of the first and second linear rods. The apparatus may further include means for adjusting the first length of the first and second linear rods, or means for increasing the first length of the first and second linear rods.
The present invention further discloses a method for calibrating the position of a wafer platform in an ion implanter which can be carried out by the operating steps of first providing an apparatus the includes a curvilinear piece formed of a first rigid material, the curvilinear piece has a half-circular shape, a predetermined thickness, a predetermined depth, an inside diameter, an inside peripheral surface, a first open end and a second open end; a first and a second linear rod formed of a second rigid material and has equally a first length, a rectangular cross-section, two front ends and two backends, the two front ends have a thickness substantially equal to the predetermined thickness of the curvilinear piece for mounting the curvilinear piece thereon by engaging the first and second open ends of the curvilinear piece, the curvilinear piece has a plane along radial direction that is perpendicular to a plane of the first and second linear rod along a longitudinal direction when the curvilinear piece is mounted to the first and second linear rod, and at least one cross-bracing rod connected in-between and adjacent to the two backends of the first and second linear rods for providing rigidity of the apparatus; and mounting the curvilinear piece on top of the wafer platform such that the inside peripheral surface intimately engages an outside peripheral surface of the wafer platform and verifying the wafer platform is properly calibrated at a zero-angle position.
The method for calibrating the position of a wafer platform in an ion implanter may further include the step of forming the curvilinear piece and the first and second linear rods in a material selected from the group consisting of aluminum, stainless steel and fiber enforced nylon. The method may further include the step of providing two cross-bracing rods connecting in-between the first and second linear rods with one of the two cross-bracing rods positioned juxtaposed to the two backends of the first and second linear rods, or the step of mounting a length-adjusting means onto the first and second linear rods, or the step of increasing the first length of the first and second linear rods.
The present invention is further directed to a method for calibrating the position of a wafer platform in an ion implanter which can be carried out by the operating steps of first providing an apparatus that includes a curvilinear piece formed of a first rigid material, the curvilinear piece has a half-circular shape, a predetermined thickness, a predetermined depth, an inside diameter, an inside peripheral surface, a first open end and a second open end, a first and a second linear rod formed of a second rigid material and has equally a first length, a rectangular cross-section, two front ends and two backends, the two front ends have a thickness substantially equal to the predetermined thickness of the curvilinear piece for mounting the curvilinear piece thereon by engaging the first and second open end of the curvilinear piece, the curvilinear piece has a plane along radial direction that is perpendicular to a plane of the first and second linear rod along a longitudinal direction when the curvilinear piece is mounted to the first and second linear rod, and at least one cross-bracing rod connected in-between and juxtaposed to the two backends of the first and second linear rod for providing rigidity of the apparatus; and mounting the curvilinear piece on top of the wafer platform such that the first and second linear rod fit snugly with a spacing of less than 0.5 mm on the implanter indicative of a proper calibration at a zero-angle position for the wafer platform.